The probable exoplanet candidate is found in Messier 51 (M51), which is also known as the Whirlpool Galaxy due to its unusual profile.
Planets outside of our Solar System are known as exoplanets. Until now, astronomers have discovered all known exoplanets and exoplanet candidates in the Milky Way galaxy, nearly all of them within 3,000 light-years of Earth. Exoplanets in M51 would be 28 million light-years away, thousands of times farther away than those in the Milky Way.
“By searching for planet candidates at X-ray wavelengths, we’re hoping to open up a whole new arena for finding other worlds,” said Rosanne Di Stefano of the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Massachusetts, who led the work, which was published in Nature Astronomy.
This new discovery is based on transits, which occur when a planet passes in front of a star, blocking some of the star’s light and causing a distinctive dip. Astronomers have searched for dips in optical light, electromagnetic radiation that humans can see, using both ground-based and space-based observatories, such as those on NASA‘s Kepler and TESS missions. This has led to the finding of thousands of planets.
Instead, Di Stefano and colleagues looked for dips in the brightness of X-rays emitted by X-ray bright binaries. A neutron star or black hole pulls gas in from a closely orbiting companion star in these bright systems. Near a neutron star or black hole, material becomes superheated and glows in X-rays.
Because the zone producing bright X-rays is small, a planet passing in front of it might block most or all of the X-rays, making the transit easier to identify. This could allow exoplanets to be discovered at considerably greater distances than existing optical light transit investigations, which must be able to detect minute changes in light since the planet only blocks a portion of the star’s light.
The scientists utilised this strategy to find an exoplanet candidate in M51-ULS-1, a binary system in M51. A black hole or neutron star orbits a companion star with a mass of nearly 20 times that of the Sun in this binary system. They discovered a three-hour X-ray transit using Chandra data, during which the X-ray output dropped to zero. The researchers believe that the exoplanet candidate in M51-ULS-1 is around the size of Saturn and orbits the neutron star or black hole at nearly twice the distance Saturn orbits the Sun, based on this and other data.
While this is an intriguing study, more data is needed to confirm the interpretation of the planet as an extragalactic exoplanet. One problem is that the planet candidate’s huge orbit means it won’t cross in front of its binary partner for another 70 years, thereby putting an end to any attempts at confirmation for decades.
“Unfortunately, we’d have to wait decades to witness another transit to establish that we’re seeing a planet,” said co-author Nia Imara of the University of California in Santa Cruz. “And we wouldn’t know exactly when to look because of the uncertainties about how long it takes to orbit.”
Could a cloud of gas and dust moving in front of the X-ray source have caused the dimming? This is an implausible explanation, according to the researchers, because the characteristics of the event recorded in M51-ULS-1 are not consistent with the passage of such a cloud. The data, on the other hand, support the notion of a planet candidate.
“We know we’re making a big claim, so we expect other astronomers to scrutinise it closely,” said Julia Berndtsson of Princeton University in New Jersey, a co-author. “We believe we have a compelling case, and this is how science works.”
If there is a planet in this system, it has most certainly had a turbulent history and violent past. A supernova explosion that formed the neutron star or black hole would have required an exoplanet in the system to survive. The future could be hazardous as well. The partner star could also explode as a supernova at some point, blasting the planet with extraordinarily high amounts of radiation once more.
Di Stefano and her colleagues used Chandra and the European Space Agency’s XMM-Newton to search for X-ray transits in three galaxies beyond the Milky Way galaxy. Their search included 55 M51 systems, 64 Messier 101 (the “Pinwheel”) systems, and 119 Messier 104 (the “Sombrero”) systems, yielding the solitary exoplanet candidate detailed here.
The researchers will look through the Chandra and XMM-Newton archives for other exoplanet candidates in other galaxies. At least 20 galaxies have large Chandra datasets, including those that are significantly closer than M51, such as M31 and M33, allowing for the detection of shorter transits. Another intriguing area of inquiry is looking for X-ray transits in Milky Way X-ray sources in order to find new planets in strange surroundings.
Ryan Urquhart (Michigan State University), Roberto Soria (University of the Chinese Science Academy), Vinay Kashap (CfA), and Theron Carmichael (Michigan State University) are the other authors of this Nature Astronomy paper (CfA). The Chandra programme is overseen by NASA’s Marshall Space Flight Center. The Chandra X-ray Center of the Smithsonian Astrophysical Observatory manages science from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
Source- Center for Astrophysics, Harvard & Smithsonian
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